Lossy compressed audio formats have been known for over a decade, and audio devices capable of playing back content encoded in lossy compressed audio formats have been available for over half a decade. Lossy compressed audio formats overcame limitations associated with computers and networks as audio playback environments. In particular, with the advent of optical disks for program storage and distribution, it became apparent that audio playback capability based on compact disks could easily be added to desktop computers.
Those using optical disk drives incorporated in desktop computers as audio playback devices quickly realized the limitations of the hardware. Early optical disk drives were expensive, and whenever an optical disk needed to be read or written for productivity purposes, it required that an audio disk (if in use) to be removed from the optical disk drive. In order to overcome this limitation, it was realized that audio content could be stored on a hard drive. No longer would it be necessary to interrupt audio playback while performing productivity operations that required use of an optical drive. However, those familiar with the situation realized that current hard drives were not practical as media for storing audio encoded at the bit rate reflected in the compact disk format.
Conventional compact disks encoding audio information typically store anywhere from 300 to 700 mbytes of information. Hard drives available in the mid- to late-1990s were simply of too-limited capacity to store significant amounts of audio information encoded in the compact disk format, especially when those interested in doing so realized that a desktop computer could be used as a “jukebox”. In order to overcome this limitation, it became apparent that new encoding formats needed to be developed that would result in a significant decrease in file sizes.
The MP3 format was developed to accomplish this. During development of the MP3 encoding format, it was realized that in a passage of music, certain elements occurring in close proximity time-wise to other elements would mask those other elements from a human listener. Once this phenomenon of human hearing was recognized, those seeking greater compression of audio information realized that lossy encoding formats could be adopted. Such lossy formats would save file space by not encoding information associated with content that was effectively masked to human listeners. Resulting lossy formats, like the MP3 standard, achieve a many-fold or more decrease in file size while maintaining reasonable audio quality.
The situation has changed, though, with the advent of terabyte hard drives and wide-band wired and wireless communications networks. Particularly with respect to desktop computers, it is no longer necessary to employ lossy audio encoding formats since a large-capacity hard drive can easily accommodate all of a user's compact disks with room left over, even if the user's disk collection extends to hundreds of compact disks. Thus, lossless encoding capability has been added to well-known music management and playback software packages.
A frequent complaint heard concerning on-line music stores is that music content is available only in lossy, low bit-rate formats. In view of the fact that many users have access to wideband network connections, those users demand access to higher-quality encoding formats, up to and including lossless encoding formats. Alternatively, users may not always desire higher-quality music associated with high bitrates. For instance, portable music players typically have much-smaller hard drives when compared to desktop computers. In such instances, it becomes necessary to transcode a music collection encoded at a high bit rate to a low bitrate if the music collection is to “fit” on the hard drive of the portable music player.
In addition, transmission of high-quality audio content occurs in some situations over a package-switched network that does not provide perfect quality of service. In such situations, it can be expected that packets encoding audio information will be dropped. In other content distribution situations, users may have playback devices with varying capability, or users may desire varying levels of audio fidelity. In such situations, it would be impractical to provide each user with bitstreams of audio content at the user's desired bit rate.
To accommodate these varying playback environments, scalable methods of encoding audio information have been developed. Such methods encode information at high bit rates, but permit the audio information to be decoded at lower bit rates. For example, audio content encoded in a lossless format can be decoded in lossy formats at varying rates like 128 kbit/s; 96 kbit/s; 64 kbit/s or 32 kbit/s. Such an approach is highly efficient. Although large-capacity hard drives have become available, it would still be economically inefficient to store multiple copies of an audio file at different bit rates. Instead, it is far more efficient to encode an audio file in an encoding format that supports fine-grain bitrate scalability, enabling, e.g., the transmission of a single bitstream that may be decoded ay many varying rates.
Concurrently with these developments, the search for more efficient codecs for encoding audio information continues. Once such encoding method creates compressed audio data using companding and vector quantization of frequency domain coefficients representing the audio data. This method has proved advantageous in comparison to other encoding methods.
In view of the advantages of compression methods using companding and vector quantization, those skilled in the art seek to expand the usefulness of these methods by combining them with scalable encoding methods.